The present invention relates to a coating for cables which is capable of protecting the cable from accidental impacts.
Accidental impacts on a cable, which may occur, for example, during their transportation, laying etc., may cause a series of structural damage to the cable, including deformation of the insulating layer, detachment of the insulating layer from the semiconductive layer, and the like; this damage may cause variations in the electrical gradient of the insulating coating, with a consequent decrease in the insulating capacity of this coating.
In the cables which are currently commercially available, for example in those for low- or medium-tension power transmission or distribution, metal armor capable of withstanding such impacts is usually applied in order to protect cables from possible damages caused by accidental impacts. This armor may be in the form of tapes or wires (generally made of steel), or alternatively in the form of a metal sheath (generally made of lead or aluminum); this armor is, in turn, usually clad with an outer polymer sheath. An example of such a cable structure is described in U.S. Pat. No. 5,153,381.
The Applicant has observed that the presence of the abovementioned metal armor has a certain number of drawbacks. For example, the application of the said armor includes one or more additional phases in the processing of the cable. Moreover, the presence of the metal armor increases the weight of the cable considerably, in addition to posing environmental problems since, if it needs to be replaced, a cable constructed in this way is not easy to dispose of.
The Japanese patent published under the number (Kokai) 7-320550 describes a domestic cable with an impact-resistant coating 0.2-1.4 mm in thickness, placed between the insulator and the outer sheath. This impact-resistant coating is a non-expanded polymer material containing a polyurethane resin as main component.
On the other hand, use of expanded polymeric materials in cables"" construction is known for a variety of purposes.
For instance, German patent application no. P 15 15 709 discloses the use of an intermediate layer between the outer plastic sheath and the inner metallic sheath of a cable, in order to increase the resistance of the outer plastic sheath to low temperatures. No mention is made in such document about protecting the inner structure of the cable with said intermediate layer. As a mattter of fact, such intermediate layer should compensate for elastic tensions generated in the outer plastic sheath due to temperature""s lowering and may consist of loosely disposed glass fibers or of a material which may either be expanded or incorporating hollow glass spheres.
Another document, German utility model no. G 81 03 947.6, discloses an electric cable for use in connections inside apparatuses and machines, having particular mechanical resistance and flexibility. Said cable is specifically designed for passing on a pulley and is sufficiently flexible in order to recover its straight structure after the passage on said pulley. Accordingly, this kind of cable is specifically aimed to resist to mechanical loads of the static type (such as those generated during the passage onto a pulley), and its main feature is the flexibility. It is readily apparent to those skilled in the art that this kind of cable substantially differs from low- or medium-tension power transmission or distribution having a metal armor which, rather to be flexible, should be capable of withstanding dynamic loads due to impacts of a certain strength onto the cable.
In addition, in signal transmission cables of the coaxial or twisted pair type, it is known to use expanded materials in order to insulate a conductive metal. Coaxial cables are usually intended to carry high-frequency signals, such as coaxial cables for TV (CATV) (10-100 MHz), satellite cables (up to 2 GHz), coaxial cables for computers (above 1 MHz); traditional telephone cables usually carry signals with frequencies of about 800 Hz.
The purpose of using an expanded insulator in such cables is to increase the transmission speed of the electrical signals, in order to approach the ideal speed of signal transmission in an aerial conductive metal (which is close to the speed of light). The reason for this is that, compared with non-expanded polymer materials, expanded materials generally have a lower dielectric constant (K), which is proportionately closer to that of air (K=1) the higher the degree of expansion of the polymer.
For example, U.S. Pat. No. 4,711,811 describes a signal transmission cable having an expanded fluoropolymer as insulator (thickness of 0.05-0.76 mm) clad with a film of ethylene/tetrafluoroethylene or ethylene/chlorotrifluoroethylene copolymer (thickness of 0.013-0.254 mm). As described in that patent, the purpose of the expanded polymer is to insulate the conductor, while the purpose of the film of non-expanded polymer which clads the expanded polymer is to improve the mechanical properties of the insulation, in particular by imparting the necessary compression strength when two insulated conductors are twisted to form the so-called xe2x80x9ctwisted pairxe2x80x9d.
Patent EP 442,346 describes a signal transmission cable with an insulating layer based on expanded polymer, placed directly around the conductor; this expanded polymer has an ultramicrocellular structure with a void volume of greater than 75% (corresponding to a degree of expansion of greater than 300%). The ultramicrocellular structure of this polymer should be such that it is compressed by at least 10% under a load of 6.89xc3x97104 Pa and recovers at least 50% of its original volume after removal of the load; these values correspond approximately to the typical compression strength values which the material needs to have in order to withstand the compression during twisting of the cables.
In International patent application WO 93/15512, which also relates to a signal transmission cable with an expanded insulating coating, it is stated that by coating the expanded insulator with a layer of non-expanded insulating thermoplastic polymer (as described, for example, in the abovementioned U.S. Pat. No. 4,711,811) the required compression strength is obtained, this however reducing the speed of propagation of the signal. The said patent application WO 93/15512 describes a coaxial cable with a double layer of insulating coating, where both the layers consist of an expanded polymer material, the inner layer consisting of microporous polytetrafluoroethylene (PTFE) and the outer layer consisting of a closed-cell expanded polymer, in particular perfluoroalkoxytetrafluoroethylene (PFA) polymers. The insulating coating based on expanded polymer is obtained by extruding the PFA polymer over the inner layer of PTFE insulator, injecting Freon 113 gas as expanding agent. According to the details given in the description, this closed-cell expanded insulator makes it possible to maintain a high speed of signal transmission. It is moreover defined in that patent application as being resistant to compression, although no numerical data regarding this compression strength are given. The description emphasizes the fact that conductors clad with such a double-layer insulator can be twisted. Moreover, according to that patent application, the increase in void volume in the outer expanded layer makes it possible to obtain an increase in the speed of transmission, thereby giving rise to small variations in the capacity of this coating to oppose the compression of the inner expanded layer.
As is seen from the abovementioned documents, the main purpose of using xe2x80x9copen cellxe2x80x9d expanded polymer materials as insulating coatings for signal transmission cables is to increase the speed of transmission of the electrical signal; however, these expanded coatings have the drawback of having an insufficient compression strength. A few expanded materials are also generically defined as xe2x80x9cresistant to compressionxe2x80x9d, since they have to ensure not only a high speed of signal transmission but also a sufficient resistance to the compression forces which are typically generated when two conductors coated with the abovementioned expanded insulation are twisted together; accordingly, also in this case, the applied load is substantiantially of static type.
Thus, while, on the one hand, it is necessary for these insulating coatings made of expanded polymer material for signal transmission cables to have characteristics such that they can bear a relatively modest compression load (such as that which arises when two cables are twisted together), on the other hand, no mention is made in any document known to the Applicant of any type of impact strength which may be provided by an expanded polymer coating. Moreover, although such an expanded insulating coating promotes a higher speed of signal transmission, this is considered to be less advantageous than a coating made of a similar non-expanded material as regards the compression strength, as reported in the abovementioned patent application WO 93/15512.
The Applicant has now found that by inserting into the structure of a power transmission cable a suitable coating made of expanded polymer material of adequate thickness and flexural modulus, preferably in contact with the sheath of outer polymer coating, it is possible to obtain a cable having a high impact strength, thereby making it possible to avoid the use of the abovementioned protective metal armor in the structure of this cable. In particular, the Applicant has observed that the polymer material should be selected in order to have a sufficiently high flexural modulus, measured before its expansion, so to achieve the desired impact resistant properties and avoid possible damages of the inner structure of the cable due to undesired impacts on the outer surface of it. In the present description, the term xe2x80x9cimpactxe2x80x9d is intended to encompass all those dynamic loads of a certain energy capable to produce substantial damages to the structure of conventional unarmored cables, while while having negligible effects on the structure of conventional armored cables. As an indication, such an impact may be considered an impact of about 20-30 joule produced by a V-shaped rounded-edge punch, having a curvature radius of about 1 mm, onto the outer sheath of the cable.
The Applicant has moreover observed that, surprisingly, an expanded polymer material used as a coating for cables according to the invention makes it possible to obtain an impact strength which is better than that obtained using a similar coating based on the same polymer which is not expanded.
A cable with a coating of this type has various advantages over a conventional cable with metal armor such as, for example, easier processing, a reduction in the weight and dimensions of the finished cable and a reduced environmental impact as regards recycling of the cable once its working cycle is over.
One aspect of the present invention thus relates to a power transmission cable comprising
a) a conductor;
b) at least one layer of compact insulating coating,
c) a coating made of expanded polymer material, wherein said polymer material has predetermined mechanical strength properties and a predetermined degree of expansion so as to impart impact resistant properties to said cable.
According to a preferred aspect of the present invention, the expanded polymer material is obtained from a polymer material which has, before expansion, a flexural modulus at room temperature, measured according to ASTM standard D790, higher than 200 MPa, preferably between 400 MPa and 1500 MPa, values of between 600 MPa and 1300 MPa being particularly preferred.
According to a preferred aspect, said polymer material has a degree of expansion of from abuot 20% to about 3000%, preferably from about 30% to about 500%, a degree of expansion of from about 50% to about 200% being particularly preferred.
According to a preferred embodiment of the present invention, the coating of expanded polymer material has a thickness of at least 0.5 mm, preferably between 1 and 6 mm, in particular between 2 and 4 mm. According to a preferred aspect of the present invention, this expanded polymer material is chosen from polyethylene (PE), low density PE (LDPE), medium density PE (MDPE), high density PE (HDPE) and linear low density PE (LLDPE); polypropylene (PP); ethylene-propylene rubber (EPR), ethylene-propylene copolymer (EPM), ethylene-propylene-diene terpolymer (EPDM); natural rubber; butyl rubber; ethylene/vinyl acetate (EVA) copolymer; polystyrene; ethylene/acrylate copolymer, ethylene/methyl acrylate (EMA) copolymer, ethylene/ethyl acrylate (EEA) copolymer, ethylene/butyl acrylate (EBA) copolymer; ethylene/xcex1-olefin copolymer; acrylonitrile-butadiene-styrene (ABS) resins; halogenated polymer, polyvinyl chloride (PVC); polyurethane (PUR); polyamide; aromatic polyester, polyethylene terephthalate (PET), polybutylene terephthalate (PBT); and copolymers or mechanical mixtures thereof.
According to a further preferred aspect, this polymer material is a polyolefin polymer or copolymer based on PE and/or PP, preferably modified with ethylene-propylene rubber, in which the PP/EPR weight ratio is between 90/10 and 50/50, preferably between 85/15 and 60/40, in particular about 70/30.
According to a further preferred aspect, this polyolefin polymer or copolymer based on PE and/or PP contains a predetermined amount of vulcanized rubber in powder form, preferably between 10% and 60% of the weight of the polymer.
According to a further preferred aspect, this cable moreover comprises an outer polymer sheath, which is preferably in contact with the expanded polymer coating, this sheath preferably having a thickness of at least 0.5 mm, preferably between 1 and 5 mm.
Another aspect of the present invention relates to a method for imparting impact strength to a cable, which comprises coating this cable with a coating made of expanded polymer material.
According to a preferred aspect, this method for imparting impact strength to a cable moreover comprises coating this expanded coating with an outer protective sheath.
A further aspect of the present invention relates to the use of an expanded polymer material in order to impart impact strength to a power transmission cable.
A further aspect of the present invention relates to a method for evaluating the impact strength of a cable comprising at least one insulating coating, this method consisting in
a) measuring the average peel strength of the said insulating layer;
b) subjecting the cable to an impact of predetermined energy;
c) measuring the peel strength of the said insulating layer at the point of impact;
d) checking that the difference between the average peel strength and the peel strength measured at the point of impact is less than a predetermined value for the said cable relative to the average peel strength.
According to a preferred aspect, this peel strength is measured between the layer of insulating coating and the outer layer of semiconductive coating.
In the present description, the term xe2x80x9cdegree of expansion of the polymerxe2x80x9d is understood to refer to the expansion of the polymer determined in the following way:
G (degree of expansion)=(d0/dexe2x88x921)xc2x7100 
where d0 indicates the density of the non-expanded polymer (that is to say the polymer with a structure which is essentially free of void volume) and de indicates the apparent density measured for the expanded polymer.
For the purposes of the present description, the term xe2x80x9cexpandedxe2x80x9d polymer is understood to refer to a polymer within the structure of which the percentage of void volume (that is to say the space not occupied by the polymer but by a gas or air) is typically greater than 10% of the total volume of this polymer.
In the present description, the term xe2x80x9cpeelxe2x80x9d strength is understood to refer to the force required to separate (peel) a layer of coating from the conductor or from another layer of coating; in the case of separation of two layers of coating from each other, these layers are typically the insulating layer and the outer semiconductive layer.
Typically, the insulating layer of power transmission cables has a dielectric constant (K) of greater than 2. Moreover, in contrast with signal transmission cables, in which the xe2x80x9celectrical gradientxe2x80x9d parameter does not assume any importance, electrical gradients ranging from about 0.5 kV/mm for low tension, up to about 10 kV/mm for high tension, are applied in power transmission cables; thus, in these cables, the presence of inhomogeneity in the insulating coating (for example void volumes), which could give rise to a local variation in the dielectric rigidity with a consequent decrease in the insulating capacity, tends to be avoided. This insulating material will thus typically be a compact polymer material, in which, in the present description, the term xe2x80x9ccompactxe2x80x9d insulator is understood to refer to an insulating material which has a dielectric rigidity of at least 5 kV/mm, preferably greater than 10 kV/mm, in particular greater than 40 kV/mm for medium-high tension power transmission cables. In contrast with an expanded polymer material, this compact material is substantially free of void volume within its structure; in particular, this material will have a density of 0.85 g/cm3 or more.
In the present description, the term low tension is understood to refer to a tension of up to 1000 V (typically greater than 100 V), the term medium tension is understood to refer to a tension from about 1 to about 30 kV and the term high tension is understood to refer to a tension above 30 kV. Such power transmission cables typically operate at nominal frequencies of 50 or 60 Hz.
Although, in the course of the description, the use of the expanded polymer coating is illustrated in detail with reference to power transmission cables, in which this coating may advantageously replace the metal armor currently used in such cables, it is clear to those skilled in the art that this expanded coating may advantageously be used in any type of cable for which it might be desired to impart suitable impact protection to such a cable. In particular, the definition of power transmission cables includes not only those specifically of the type for low and medium tension but also cables for high-tension power transmission.